The present invention pertains to furnaces, and more particularly to a microprocessor control system and method for providing high and low heating rates in a furnace utilizing a single stage thermostat.
Presently, most furnace controls include a two-stage thermostat for providing dual rate heating, wherein the first stage of the thermostat operates the furnace on low heat and the second stage of the thermostat operates the furnace on high heat. For example, a typical two-stage thermostat comprises two small mercury bulb contacts on a bi-metal sensor that close and open as a function of the movement of the bi-metal sensor in response to changing temperatures. If the present temperature is below a desired temperature setpoint, for example, one degree to two degrees below the setpoint, then the low heat mercury bulb contact closes to provide low heat. If the temperature continues to drop to, for example, three degrees below the desired temperature setpoint, then the second mercury bulb contact closes to provide a high rate of heat. When the heating load is satisfied, both of the mercury bulb contacts will sequentially open or deactivate, thereby terminating the heating cycle. Thereafter, when the thermostat contacts close indicating a new heating load to be satisfied, then the furnace control repeats the same heating cycle all over again.
Several disadvantages of the above type of furnace control include the requirement of a two-stage thermostat for providing dual rates of heat and large room temperature swings.